1. Field of the Invention
This invention relates to an inverter transformer to which a balance transformer is integrally assembled, and a discharge tube drive circuit using such inverter transformer.
2. Description of the Related Art
It is well-known technology in a cold cathode discharge tube drive circuit that a balance coil is connected to a low voltage side of a drive circuit or a balance coil is connected to a high voltage side to which a cold cathode discharge tube of the drive circuit is connected in order to control current flowing through a plurality of discharge tubes to be constant. In the cold cathode discharge tube, if their impedances fluctuate, the voltage applied across both electrodes of the cold cathode discharge tube also fluctuates. Accordingly, it is known that the current flowing through each of the cold cathode discharge tube changes depends on the impedance of each of the cold cathode discharge tubes. If the current flowing through the cold cathode discharge tube fluctuates, this effects light emission of the cold cathode discharge tubes.
As well-known, a plurality of cold cathode discharge tubes are provided in a backside of a LCD panel as back-light. As a recent trend, a display size of the LCD panel becomes larger. For example, a home-use LCD-TV have had 20 inches, however, recently, LCD-TV using LCD panel having display size of 32 or 34 inches becomes popular. As the size of the LCD panel becomes larger, the number of necessary discharge tubes per one LCD-TV also increases.
As described above, if each amount of current flowing through each of cold cathode discharge tubes is different from each other, unevenness of light emission among cold cathode discharge tubes, and this causes unevenness of luminance level in a LCD panel. Accordingly, it is positively necessary to adjust the amount of current flowing thorough each cold cathode discharge tubes to be constant. Therefore, normally, an inverter transformer and a balance transformer are employed in a drive circuit for cold cathode discharge tubes.
Conventionally, it is proposed to a method in which a balance coil is connected to a low voltage side or a high voltage side of a cold cathode discharge tube drive circuit. Further, it is also proposed to provide a plurality of corresponding coils in order to apply to a plurality of cold cathode discharge tubes with a single magnetic path of a balance transformer such as disclosed in Japanese Patent Laid-open 2003-31383. In addition, it is proposed a circuit in Published Japanese translation of PCT International Publication for patent application 2004-506294 wherein first inductor connected to primary coil of a first transformer and second inductor connected to primary coil of second transformer are provided in the same magnetic path. Further, there is provided an inverter transformer configuring with 1, H, and I cores in Japanese Patent Laid-open 2005-311227. In addition, there are also provided a transformer in which a main transformer and a signal transmission transformer are integrally assembled and a transformer in which a choke coil and a transformer are integrally assembled as disclosed in Japanese Patent Laid-open 2004-349293, 64-030463, and 2000-133531.